Exposed photographic materials, for example black-and-white photographic materials, are developed in a solution which reduces the exposed silver halide. The unexposed silver halide is then removed by treating the film or paper with a second solution known as a fixer which comprises a silver halide complexing agent, e.g., a thiosulfate. The fixer solution is acidic to ensure that any developer which is carried over from the development stage does not remain active. If the fixer solution is insufficiently acidic, the silver complexes formed on fixing can be reduced by the active developer to form dichroic fog.
It is essential that the pH of the photographic material is reduced rapidly throughout the emulsion layer when fixing takes place. In order to achieve this, a fixer which is buffered between pH 4 and 5 may be used. If the fixer is not buffered then either it will not be able to neutralize the alkali quickly in the emulsion or it will require a very low pH making it unsafe and unstable.
It is known to use an organic aliphatic acid as a buffer at a concentration between 1 and 3% in the working strength fixer solution as it will buffer in the region of pH 3 to 5. The acid most commonly used is acetic acid but other weak acids have also been used, particularly when they have less odor, e.g., tartaric, succinic and citric acids.
JP-A-5127323 describes a fixing composition having reduced odor comprising a fixing agent, e.g., ammonium thiosulfate, a stabilizer, e.g., sodium sulfite, a hardening agent, e.g., aluminum sulfate and acids including succinic or maleic acid.
The odor associated with fixers also appears to be due to the evolution of sulfur dioxide produced by the disproportionation of sulfite and the fixing agent in acid. The odor problem caused by the evolution of sulfur dioxide needs to be overcome.
Although it has been found that sufficiently raising the pH of the fixer can substantially reduce or eliminate the evolution of sulfur dioxide, the fixer loses its ability to effectively neutralize the alkaline solution carried over by the photographic material during processing. A typical fixer concentrate has a pH drop of about 0.1 unit on dilution with water to the working strength fixer. If the odor problem of the fixer concentrate is to be overcome by increasing pH, a much greater drop in pH is required on dilution with water.